Galactosemia is an autosomal recessive disorder caused by a defect in any one of the three enzymes comprising the Leloir pathway, galactokinase (GALK), galactose-1-phosphate uridylyltransferase (GALT), and UDPgalactose-4'-epimerase (GALE). Of the three disorders, GALT-deficiency galactosemia is currently considered to be the most common and clinically significant, with an incidence rate of 1 out of 40,000 live births in the United States. GALE deficiency results in a range of symptomsfrom benign to severe and is not well characterized. Fortunately, early diagnosis by newborn screening programs and intervention helps to prevent the acute toxicity and potentially life-threatening complications of the disease. Life-long dietary restriction of galactose is the current treatment for either GALT or severe GALE deficiency. However, despite long-term galactose restriction, a large fraction of patients still experience significant clinical complications. The biochemical mechanism underlying the pathology in galactosemia remains unknown, limiting possibilities for the generation of novel and potentially more effective therapies. The functional and metabolic consequences of impaired galactose metabolism in both Saccharomyces cerevisiae and mammalian cells will be studied in this proposal to define further the specific mediators and biochemical abnormalities that occur in galactosemia.